SABnzbd compatibility: POST mode=addurl fails with 400 when request has no multipart body

[RegWin32]

SABnzbd compatibility: POST mode=addurl fails without multipart body

Prowlarr sends SABnzbd mode=addurl requests as a POST request with all parameters in the query string and no multipart/form-data body.
rustnzb currently rejects this request with:

HTTP/1.1 400 Bad Request
Invalid `boundary` for `multipart/form-data` request

This prevents rustnzb from being used as a SABnzbd download client in Prowlarr for grabbing releases.

PR #70 added addurl support over GET and extracted the shared handle_addurl() logic, but the POST route still appears to require a valid multipart body before handle_addurl() can be reached.

Environment

• Prowlarr: 2.5.2.5491
• rustnzb: latest
• SABnzbd compatibility endpoint: /sabnzbd/api

Minimal reproduction

This request reproduces the problem:

curl -i -X POST \
"http://localhost:9090/sabnzbd/api?mode=addurl&name=https%3A%2F%2Fexample.com%2Ftest.nzb&cat=prowlarr&priority=-100&apikey=REDACTED&output=json"

Result:

HTTP/1.1 400 Bad Request
content-type: text/plain; charset=utf-8

Invalid `boundary` for `multipart/form-data` request

However, adding an otherwise unnecessary multipart field:

curl -i -X POST \
-F "dummy=1" \
"http://localhost:9090/sabnzbd/api?mode=addurl&name=https%3A%2F%2Fexample.com%2Ftest.nzb&cat=prowlarr&priority=-100&apikey=REDACTED&output=json"

causes the multipart boundary error to disappear and rustnzb proceeds to the URL-fetching / addurl logic.

For example, with an intentionally invalid URL this results in an application-level error instead of the multipart parsing error:

{
  "error_kind": "internal_error",
  "human_readable": "Failed to fetch URL: builder error",
  "status": 500
}

This strongly suggests that the POST handler requires the Axum Multipart extractor even for mode=addurl, despite addurl not requiring a file upload or multipart body.

Expected behavior

POST mode=addurl should work without requiring a multipart/form-data body.
Prowlarr sends the SABnzbd request in this form:

POST /sabnzbd/api?mode=addurl&name=<NZB_URL>&cat=prowlarr&priority=-100&apikey=<API_KEY>&output=json

The request parameters are already provided in the query string, so no multipart body should be required for mode=addurl.
Multipart parsing should only be required for operations that actually upload a file, such as mode=addfile.
Ideally, the existing handle_addurl() logic introduced around PR #70 could be reused for both:

GET  mode=addurl
POST mode=addurl

without requiring multipart data for the POST case.